Neuronal loss and dysfunction can arise both during development due to congenital defects, and at maturity because of injury and disease. Failure to produce the correct cell types and their precise connectivity patterns have severe functional consequences. Thus, our broad goal is to advance our understanding of: (i) the cellular processes that ensure the proper generation of neurons and their circuits during development, and (ii) the processes that enable or limit neuronal populations from re-establishing their origina circuitry after injury or disease. Although stem-cell therapy represents a major strategy for restoring function, it is not yet known whether new neurons placed in injured or diseased cellular environments are able to form their original connectivity patterns. Because survival of cells seeded within the central nervous system remains challenging, we will take advantage of zebrafish, an important genetic model system for investigating development and disease, to achieve our goals. This is because zebrafish have an inherent ability to regenerate its neurons. We propose to focus on the first synapse in the visual system between cone photoreceptors and their target bipolar cells because structure, function and connectivity of these cell types are heavily studied, and perturbations to their connections result in impaired vision. We will use genetic tools and state-of-the-art imaging approaches to answer three outstanding questions in the fields of neuronal and visual development and repair. In Aim 1, we will determine whether there are endogenous cell-genesis pathways directed at producing and regenerating specific subtypes within a single neuronal type, the cone photoreceptors. In Aim 2, we will ascertain how postsynaptic bipolar cells compensate for the absence of a preferred presynaptic cone type during development, in models of congenital disease. In Aim 3, we will selectively ablate cone photoreceptors or bipolar cells in vivo and assess the specificity and accuracy of circuit reassembly upon neuronal regeneration. Together, our findings will significantly increase our understanding of the generative and regenerative processes that are recruited in vivo to establish complex circuits, such as the cone pathways, in development and in repair.